1. Technical Field
The present invention is related to devices and fixtures for coating objects with a sprayed material. More paritcularly, but no exclusively, the invention relates to devices and methods for thermal spray processes.
2. Background Art
There are many applications in which it is desired to apply a coating to a part or form a product layer using a spray process. Such applications may include applying primers, paints, and/or other types of coatings. Once such coating for which the present invention may used, but is not limited to, is the application of a tamper resistant coating (TRC). A TRC is a layer applied to a product to provide a physical barrier to prevent inspection of, and tampering with, for example, the underlying circuitry and contents of electronic components.
Processes and systems for coating electronic circuits with protective coatings and security coatings using a thermal spray are generally known. Examples of such processes are described in U.S. Pat. Nos. 5,877,093; 6,110,537; 5,762,711; and 6,319,740 all to Heffner et al. Application of TRCs by heating a material to a molten state and spraying the molten material where desired, is referred to herein as a “thermal spray” or “molten spray” process.”
FIG. 1 illustrates a thermal spray system disclosed by U.S. Pat. No. 6,110,537 to Heffner et al., which is incorporated herein by its reference. Particles of a coating material 60 are supplied from a feedstock supply to a thermal spray gun 31. Fuel and oxygen are supplied to gun 31 to heat the coating material 60 to a molten state. Air 66 is combined with the stream of molten particles and output through flame front 67 toward one or more parts or circuits mounted on arms 68. During the thermal spray process, arms 68 are rotated by motor 70. A coolant 74 may be pumped through the interior of arms 68 to regulate temperatures during the thermal spray process.
Arms 68 rotate at in the range of approximately one thousand revolutions per minute to repetitively sweep parts or circuits, e.g. attached at ends of arms 68, through the flame spray. With each pass, a layer of coating builds up on the exposed surface of an integrated circuit.
As shown in FIG. 2, U.S. Pat. No. 6,110,537 to Heffner et al. also discloses an embodiment wherein in lieu of arms 68, multiple integrated circuits 13 may be clamped in an aluminum disk fixture including disk 88 and mask 90 to a stand 92 having alignment pins 92a. Integrated circuits 13 are inserted and individually clamped in place by screws (not shown) that are inserted in holes 88b and 90a. Like arm 68 disk 88 contains internal coolant outlets 91 that connect with internal coolant passages 91a. The disk disclosed by Heffner et al. enables thermal spray processing of multiple ICs but requires a person to individually insert each circuit 13 in the disk before performing the thermal spray process. Furthermore, each circuit must be individually removed from the disk by hand before further processing can be performed, if any is required.
The disk approach disclosed by Heffner et al. has subsequently evolved into the use of larger wheels and device holding structures mounted thereto. With a larger wheel, a plurality of device-holding fixtures can be mounted and processed. Further, spray guns mounted on movable arms were incorporated to cover the increasing spray area on the larger spray target wheels.
Placement of parts in fixtures along a circumference of a wheel maximized cooling efficiently at the expense of coating efficiency. Much of the area in the circumference of the spray area did not hold parts and thus coating material was wasted and throughput limited. Attempts were then made to fill in empty spaces of the wheel to improve coating efficiency. However, these wheels and accompanying device (IC) holding fixture designs had significant disadvantages.
One disadvantage of early device-holding fixtures was that they were large and difficult to mount to the spray wheel; particularly for fixtures that may have warped through normal and continuous use. In the past, fixtures were typically mounted by placing the fixture over shoulder bolts attached to the wheel, sliding the fixture to align mounting holes and inserting a lock pin. This mounting operation often proved difficult due to misalignments between the fixture and the wheel, and consequently, extraordinary measures were often required to insert the pin.
Another disadvantage of prior art systems is that the larger spray areas made it difficult to control the quality of the applied coatings. Because the thermal spray process is sensitive to the device substrate temperatures, a large spray area requires the torch to traverse a large area of the wheel and leads to a temperature gradient; often resulting in an undesirable variation of device coatings.
Additionally, the prior art thermal spray systems often use complicated cooling systems which increase overall costs. For example, incorporating coolant passages in the wheel and/or fixtures or using force-air cooling supplied from a compressed air source adds significant costs to the overall system and consequently, the devices fabricated using such a system.
Another problem of the prior art device-holding fixtures, e.g., 88, 90 FIG. 2, were that they were largely and oddly shaped to fit an arc or segment of a circular wheel. These custom shapes had two main problems: (i) the large pie-shaped segments did not lend themselves to pre or post coating automation steps since most automation equipment handle only regular shapes such as rectangles and squares; and (ii) the irregular shapes required additional or extravagant machining and wasted stock material. Both of these problems further increased the overall costs of the system and/or device fabrication.
Yet another problem with the prior art fixtures were that assembly the fixture assemblies required screws, bolts or other threaded fasteners that: (i) required significant operator time to assemble and disassemble; and (ii) exposed the devices to high risk from mishandled fasteners dropped onto the devices thus damaging die surfaces or wire bonds.
Lastly, the device-holding fixtures of the prior art were designed for hand placement of individual product into the fixture. The requirements of this labor-intensive process delays throughput and increases operating costs.